Carrier recovery in vestigial sideband data receivers

ABSTRACT

A coherent demodulating carrier-wave recovery arrangement for a digital data communication system using vestigial-sideband amplitude-modulation techniques responds to a quadrature component only of a transmitted carrier wave. The transmitted quadrature carrier component eliminates the normal requirement for suppression of in-phase signal energy in the vicinity of zero frequency (direct current) or transmission of out-of-band pilot tones. Separate phase-locked loops control demodulating carrier frequency and phase to compensate for the transmission impairments of frequency offset and phase jitter occurring in distorting transmission channels.

United States Patent 1191 [451 Nov. 19, 1974 CARRIER RECOVERY INVESTIGIAL 3,701,023 10/1972 Fang 325/329 SIDEBAND DATA RECEIVERS PrimaryExaminer-Benedict V. Safourek [75] Inventor. llgdgnond Yu-Shang Ho,Enghshtown, Attorney, Agent, or P. Keams [73] Assignee: Bell TelephoneLaboratories, [57] ABSTRACT Incorporated Murray A coherent demodulatingcarrier-wave recovery ar- [22] Filed: June 20, 1973 rangement for adigital data communication system using vestigial-sidebandamplitude-modulation tech- [211 App! '8 niques responds to a quadraturecomponent only of a I v transmitted carrier wave. The transmittedquadrature [52] US. Cl 325/329, 325/419, 329/50 carrier componenteliminates the normal requirement [51] Int. Cl. H04b 1/30 forsuppression of in-phase signal energy in the vicin- [58] Field of Search325/49, 50, 60, 63, 329, ity of zero frequency (direct current) ortransmission 325/330, 331, 418, 419, 420, 421, 422, 423; of out-of-bandpilot tones. Separate phase-locked 329/50, 122, 124 loops controldemodulating carrier frequency and phase to compensate for thetransmission impairments [56] References Cited of frequency offset andphase jitter occurring in dis- UNTED STATES PATENTS torting transmissionchannels. 3,384,824 5/1968 Grenier 325/49 4 Claims, 4 Drawing FiguresRECEIVING TERMINAL 2 Q 2| TRANSMITTING TRANSMISSION TERMINAL CHANNEL PPQE \lo 11' 12-1201-11 LOW PASS FILTER 0-3Hz CARRIER SOURCE /29 PHASE 27r LOCKED 3 COMPARATOR 3o- PHASE SHIFTER 32 l 1 31 ,26

ADDER PRODUCTOR -ATTENUATOR INTEGRATOR 36 331 35 37 38 LOW PASS IN-PHASEn DATA DEMOD f| 1 T E R SUBTRACTOn SINK CARRIER RECOVERY IN VESTIGIALSIDEBAND DATA RECEIVERS FIELD OF THE INVENTION This invention relates todemodulating carrier-wave recovery in digital data transmission systemsand specifically to the coherent detection of vestigial-sideband,amplitude-modulated data signals transmitted over band-limited channels.

BACKGROUND OF THE INVENTION Efficient use of band-limited telephonevoice channels for digital data transmission is frequently assured byusing vestigial-sideband amplitude modulation with synchronous orcoherent detection at the receiving terminal. Two commonly observedtransmission impairments, which occasion little effect on analog speechsignals, become of transcendent important when digital signals are beingtransmitted. These impairments are frequency offset and phase jitter.Frequency offset refers to the. condition wherein the demodulatingcarrier wave at the receiving terminal is not locked in frequency withthe modulating carrier wave at the transmitting terminal. This conditionupsets the harmonic relationships among the several frequency componentsof the transmitted signal. Phase jitter refers to the abrupt, spuriousvariations in phase between successive pulses as referenced to the phaseof a continuous oscillation. This condition affects the precision withwhich sampling can be accomplished.

Typically, frequency offset is no greater than three Hz for carriertelephone channels. Phase jitter appears as a low-index angle modulationof the data signal epochs at a slowly varying rate on the order of to120 Hz. Heretofore, it has been the practice to transmit along with thedata signal, pilot tones related in frequency and phase to themodulating carrier wave. The pilot tone can be the same frequency asthe'modulating wave. In this case low-frequency energy must be removedfrom the transmitted data wave and restored at the receiver to avoidinterference. On the other hand, one or more pilot tones can be locatedat the edges of the signaling band, where no data signal energy exists.In this case excess bandwidth is required in the signaling channel.

It is an object of this invention to overcome the disadvantages of theprior art in providing a demodulating carrier wave of proper phase andfrequency in a vestigi al-sideband, amplitude-modulation datatransmission system.

It is another object of this invention to track the transmissionimpairments of phase jitter and frequency offset in avestigial-sideband, amplitude-modulated digital data system to realizesmooth coherent detection without requiring band-edge pilot tones or theremoval of low-frequency energy from the data signal.

It is a further object of this invention to make more efficient use ofthe telephone voice channel for highspeed data transmission than isprovided in the prior art without materially increasing the complexityof demodulating carrier recovery systems.

SUMMARY OF THE INVENTION According to this invention, a coherent orsynchronous demodulating carrier-wave recovery arrangement for avestigial-sideband, amplitude-modulated (VSB- AM) digital datatransmission system provides continuous control of carrier phase forcoherent demodulation substantially free of the distorting effects offrequency offset and phase jitter. The received signal includes areinserted, reduced level pilot tone at the frequency of the modulatingcarrier wave. Due to the vestigialsideband signal shaping thetransmitted signal includes both in-phase and quadrature components ofthe message data wave and the carrier wave. The quadrature component ofthe data signal energy is suppressed in the vestigial-sideband filterwithout interfering with inphase data signal energy.

At the receiving terminal a demodulating carrier wave oscillator, whoseoutput is phase locked into quadrature relationship with respect to themodulating carrier wave, is controlled by a low-frequency component inthe received wave which corresponds to the frequency offset imparted intransmission, but uncorrupted by any phase jitter. The phase jittercontribution to the received wave is separately filtered from thedemodulated quadrature component and, after attenuation in proportion tothe level of the pilot tone, is product modulated with the quadraturecomponent of the locally generated carrier wave and combined with the inphase component of the locally generated carrier wave to provide ajittered in-phase demodulating carrier wave to recover a smooth basebanddata signal from the composite received signal wave. One furtheroperation is performed to remove all trace of the transmitted pilot toneafter low-pass filtering for suppression of the upper sideband anddouble frequency components. This further operation comprises theintegration of the demodulated in-phase output and subtraction of theintegrated resultant from the recovered baseband data wave. The basebanddata wave is finally converted into digital form by conventional means.

An important advantage of coherent demodulation of a vestigial-sideband,amplitude-modulated data wave in accordance with this invention is thatof bandwidth conservation without requiring dc removal and restoration.Instead of removing data signal energy in the vicinity of the frequencyof the modulating carrier wave, the transmitted in-phase componentreceives conventional Nyquist shaping for intersymbol interferenceavoidance and only the quadrature component is subjected to high-passfiltering.

A feature of this invention is that the transmission impairments offrequency offset and phase jitter frequently encountered in voicetelephone channels are compensated in substantially independent controlloops.

DESCRIPTION OF THE DRAWING The above and other objects, features andadvantages of this invention will be better appreciated by aconsideration of the following detailed description and the drawing inwhich:

FIG. 1 is a block schematic diagram of a vestigialsidebandamplitude-modulated digital data transmission system improved by acoherent demodulator according to this invention; and

FIGS. 2, 3 and 4 are frequency spectra useful in the explanation of theoperation of this invention.

DETAILED DESCRIPTION It is well known that a digital pulse train with arandom sequence of digits with discrete amplitudes a;

ture component (modulated onto the sine of the carrier frequency) asfollows:

s(t) =2; a gU-iT) cos w t-l-z a g (i-1:T) sin wet g(t) overallbandlimited signal shaping with bandwidth less than the carrierfrequency w, to avoid intersymbol interference; and

g B (t) transitional shaping within [3 Hz of the carrier frequency whichgenerates the quadrature signal component.

The shaping functions g(z) and g,, (t) are realized in a straightforwardmanner by respective low-pass and bandpass filters, each having an upperfrequency rolloff at a sampling or data transmission frequency below thecarrier frequency w The upper cutoff is conventional in datatransmission systems for avoidance of intersymbol interference. The datasequence {a,-} is applied directly to the low-pass filter having theshaping factor g(t). The data sequence {(1,} is passed through a 90allpass phase-shift circuit prior to being applied to the bandpassfilter having the shaping function g, (t), which has a low-frequencyrolloff at a frequency of ,8 Hz. The respective in-phase andquadrature-rotated data sequences, after passing through filters havingthe g(t) and g, (z) shaping functions diagrammed in FIGS. 2 and 3, aremodulated onto inphase and quadrature-phase components of the carrierfrequency in accordance with equation (1). When the modulation isbalanced, it is well known that the carrier component is eliminated fromthe output. For the purpose of facilitating coherent demodulation,however, a pilot tone having the frequency of the carrier wave isreinserted in the composite output signal at controlled amplitude, as isexplained more fully below.

The prior art use of a high-pass filter to remove direct-current energyentirely from the baseband data signal is disclosed in U.S. Pat. No.3,152,305 issued on Oct. 6, 1964 to F. K. Becker and J. R. Davey. Adirectcurrent restoration circuit is required at the receiving terminalwhen all direct-current energy is removed. According to this invention,the in-phase data signal is not subjected to low-frequency energyremoval and thus no direct-current restorer is needed.

On the assumption that the frequency offset is less than [3 Hz, theequation for the received signal r(t) with a phase jitter component (t)added by the transmission channel can be derived from equation (1) byinspection.

110:2 a,g(t-iT) cos (w,,t+ (t)) E aiUaU- Sin (OH-MU) The phase jitteramount d (t) is added to both inphase and quadrature transmitted signalcarrier waves by passage through the typical telephone channel. It iswell known that in order to demodulate the baseband data signal wavefrom the received signal defined by equation (2), a demodulating carrierwave with the LII same itter component is required, if distortion is tobe avoided. According to this invention, a VSB data signal distorted byphase jitter as represented in equation (2) and also by frequency offsetis coherently demodulated with substantially no distortion.

When frequency offset 0, is present, equation (2) becomes By slightlyincreasing the complexity ot the \SB shaping filter the quadraturecomponent of the data signal energy around zero frequency (dc) can besubstantially eliminated. On the assumption that the bandwidth of thephase jitter (t) is less than B Hz (B is also less than B), the shapingfunction g, (t) can be designed with no energy from 0 to B HzI Tnpractical telephone channels it has been found that phase jitter lies inthe range of 60 to 120 Hz. At the same time frequency offset generallyoccurs in an even lower frequency range generally not over 10 Hz.

With low-frequency components below B Hz removed from the quadraturetransmitted channel by the shaping of g a (l), but without affecting thebandwidth of the in-phas channel, the jittered received signal can P3 zrs mqss ytll.

M12 alga-m z g mu A} cos (w t-F0 +{Ei a g H-iT) ;a.g a-T)Am} in were.)(4) where A cos w is the transmitted pilot tone at the radian carrierfrequency m It is apparent that phase jitter (t) appears in thequadrature channel with the coefficient of pilot-tone amplitude A. Thequadrature channel baseband shaping is such that there is notransmission below B Hz, gradually increasing transmission to B Hz andfull transmission above B Hz. The in-phase channel baseband shaping isflat from 0 Hz to cutoff. In-phase shaping is shown in FIG. 2 as curve41. Quadrature shaping is shown in FIG. 3 as curve 43. Broken linetraces 42 in FIGS. 2 and 3 represent the g,; (t) shaping combined withthe spectrum of the half-amplitude reinserted carrier component.

FIG. 1 is a block schematic diagram of a VSB-AM digital datatransmission system modified according to this invention. Digital datasignals originating in transmission terminal 10 are shaped and modulatedonto a carrier wave to form the channel signal shown in FIG. 4 aswaveform 44 with transition step 45 of bandwidth equal to 28 Hz and anattenuated pilot tone at the carrier frequency f, at midstep. Signalshaving the waveform of FIG. 4 are conveyed over transmission channel 11to receiving tenninal 20.

Receiving terminal 20 comprises input point 21, quadrature demodulator22, in-phase demodulator 33, band-pass filter 23, low-pass filter 25,phase-locked loop comparator 29, local carrier-wave source 24,quadrature phase shifter 30, productor 31, adder 32, attenuator 26,integrator 34, low-pass filter 35, subtractor 37 and data sink 38.

Quadrature demodulator 22, low-pass filter 25, phase-locked comparator29 and carrier-wave source 24 form a tight phase-locked loop responsiveto the frequency offset 6 of the transmitted pilot tone as selected bylow-pass filter 25. Comparator 29 determines the phase differencebetween the respective outputs of carrier-wave source 24 over lead 28and filter 25 and generates a direct-current control signal to cause theoutput of carrier-wave source 24 to track the pilot tone (A cos (oincluding narrow-band frequency offset 6 The output of carrier-wavesource 24 is in quadrature.

with the transmitted pilot tone, so that the control output comparator29 is zero seeking.

The output of carrier-wave source 24 is shifted 90 in phase in phaseshifter 30 to furnish an in-phase demodulating carrier wave to in-phasedemodulator 33. However, the output of quadrature demodulator 22 is alsofiltered by bandpass filter 23 to obtain the quadrature form of phasejitter component w(t) within the frequency range of approximately 12 to120 Hz.

The output of filter 23 is the bracketed coefficient of the sine of thecarrier wave in equation (4)'shaped by the transfer characteristic offilter 23; thus where h (t) time response of filter 23 and the asteriskindicates the convolution operation.

Since g (2) contains no energy within the passband of filter 23 ,"thefirst term within the bracket is zero. The

second term is much smaller than the third term by at least 16 decibelsdue mainly to the large ratio between the data signal energy in thesecond term, typically occupying a bandwidth of 2,400 Hz, and the pilottone energy in the third term, occupying no more than 60 Hz ofbandwidth.

Accordingly, for practical purposes in implementing a voiceband datatransmission system equation (5) can be approximated by Equation (6)adequately defines for practical purposes the signal in the output ofbandpass filter 23 to be applied to attenuator 26. Assuming for themoment that the attenuation provided by attenuator 26 is l /A, oneproceeds to multiply the pure phase-jitter component from attenuator 26in productor 31 by the quadrature demodulating carrier wave providedover lead 28 from carrier-wave source 24. There results the phasejittercomponent multiplied by the sine of the frequency of the demodulatingcarrier wave, including the tion process are removed in low-pass filter35 to yield an output signal of the form {M mum we) alga-m M0) +6.

The direct-current level introduced by the presence of the pilot tone atcarrier frequency is obtained by passing the output of in-phasedemodulator 33 through integrator 34 to yield the value A of thepilot-tone amplitude appearing in equations (5), (6) and (7). When "thisvalue is subtracted from the output of filter 35 in subtractor 37, theresultant wave comprises the negligible second-order terms of thesecond-bracketed expression and the first term of the first-bracketedexpression, namely:

Equation (8) represents the transmitted data wave with the originalband-limited shaping function g(t). This analog data wave is detectedand transformed into digital baseband form in data sink 38 in aconventional manner.

The output of integrator 34 is also applied to control the level ofattenuator 26, whose purpose as previously given is to reduce the rawphase-jitter output of filter 23 by the amount of the pilot-toneamplitude A. Attenuator 26 can be implemented by a ladder network or bya field effect transistor, as disclosed, for example, in US. Pat. No.3,447,103 issued to E. Port on May 27, 1969, particularly with referenceto FIG. 4.

In summary, this invention covers a demodulating carrier recovery systemfor VSB-AM data systems. Direct-current restoration and excess bandwidthare avoided without the use of notch filters at the transmittingterminal. Only a reduced pilot tone at the carrier frequency is requiredto be transmitted in place of the two bandedge pilot tones previouslyemployed. Inphase and quadrature components of the received signal areseparately demodulated; The quadrature demodulated signal is employed intwo control loops to cause a local oscillator to track the pilot tone atcarrier frequency with respect to both frequency offset and phasejitter. The in-phase demodulating carrier wave is taken from the localoscillator through a phase shifter and has added to it a phase-jittercomponent derived from the quadrature demodulation process to serve asan in-phase demodulating carrier wave. The complexity of the VSB shapingfilter, which may be divided between transmitting and receivingterminals, is increased but slightly over the conventional filter tocause a steeper low frequency roll-off in the quadrature channel only.The specific recovery system disclosed is capable of tracking phasejitter faithfully up to 60 Hz with a relatively small 38-decibel errorin the recovered in-phase signal (the second-order terms of equation(7)).

It is to be understood that the embodiment shown and described in thisspecification is illustrative only, and that modifications may beimplemented by those skilled in the art without departing from thespirit and scope of this invention.

What is claimed is:

l. A synchronous demodulating carrier-wave recovery system for avestigial-sideband, amplitudemodulated data signal which includes adiscrete carrier component and which is received over a transmissionchannel subject to the impainnents of frequency offset and phase jittercomprising an adjustable local oscillator,

a quadrature demodulator responsive to said received signal and to theoutput of said local oscillator,

a control loop having a narrow bandwidth comparable to that of saidfrequency offset extending between said quadrature demodulator and saidoscillator to control said oscillator,

a transmission path for the output of said quadrature demodulator havinga passband comparable to that of said phase jitter for isolating aphase-jitter component therein,

means for multiplying said phase-jitter component by the output of saidlocal oscillator to form a partial in-phase demodulating signal,

means for combining said partial in-phase demodulating signal with aquadrature rotated output of said local oscillator to form a completein-phase demodulating signal having both frequency-offset andphase-jitter components, and

an in-phase demodulator responsive jointly to said received signal andto said complete in-phase demodulating signal to form a data outputsubstantially free of frequency-offset and phase-jitter impairments.

2. The synchronous carrier recovery system defined in claim 1 in whichsaid transmission path for said phase-jitter component further comprisesa bandpass filter whose bandwidth is comparable to that of saidphase-jitter component and an attenuator in series with saidlast-mentioned filter for compensating for the transmission level ofsaid discrete carrier component.

3. The synchronous carrier recovery system of claim 1 in which anintegrator operates on the output of said in-phase demodulator to derivetherefrom an output corresponding in magnitude to the transmission levelof the carrier-frequency component in said received signal and asubtractor jointly responsive to the outputs of said in-phasedemodulator and said integrator removes the direct-current componentfrom the output of said in-phase demodulator.

4. In combination with a vestigial-sideband, amplitude-modulatedtransmission system for data signals which includes a discrete carriercomponent shaped to exclude quadrature direct-current energy, saidtransmission system having a tendency to impart distortingfrequency-otfset and phase-jitter components to received signals,

a synchronous demodulating carrier recovery arrangement comprising anin-phase demodulator;

a quadrature demodulator;

means for applying received data signals to said inphase and quadraturedemodulators;

a local carrier-wave source;

a first control loop jointly controlled by said quadrature demodulatorand said carrier-wave source to lock the frequency of said carrier-wavesource to the discrete carrier component in received data signals, saidfirst control loop having a narrow passband including direct currentcomparable to the passband of said frequency-ofiset component;

a second control loop jointly responsive to said quadrature demodulatorand said carrier-wave source for tracking the phase-jitter component ofsaid received signal, said second control loop having a passband notincluding direct current comparable to the passband of said phase-jittercomponent;

means included in said second control loop for multiplying together theoutput of said carrier-wave source with said phase-jitter componentthereby producing a demodulating carrier-wave with a superposedphase-jitter component;

further means included in said second control loop for attenuating saidphase-jitter component therein in accordance with the amplitude of thediscrete carrier component in said received signal and for applying saidattenuated phase-jitter component to said multiplying means;

means for shifting the phase of the output of said carrier-wave sourceby electrical degrees to provide an in-phase demodulating carrier-wave;

means responsive to the output of said multiplying means for adding saidsuperposed phase-jitter component to the in-phase demodulating carrierwave from said phase-shifting means;

means for applying the output of said adding means to said in-phasedemodulator;

means for integrating the output of said in-phase demodulator to obtainthe direct-current component of the demodulated received signal;

means for substituting the direct-current component obtained from saidintegrating means from the demodulated received signal from saidin-phase demodulator; and

further means for applying the direct-current output from saidintegrating means to said attenuating means to control the effectivelevel of attenuation thereat.

1. A synchronous demodulating carrier-wave recovery system for avestigiaL-sideband, amplitude-modulated data signal which includes adiscrete carrier component and which is received over a transmissionchannel subject to the impairments of frequency offset and phase jittercomprising an adjustable local oscillator, a quadrature demodulatorresponsive to said received signal and to the output of said localoscillator, a control loop having a narrow bandwidth comparable to thatof said frequency offset extending between said quadrature demodulatorand said oscillator to control said oscillator, a transmission path forthe output of said quadrature demodulator having a passband comparableto that of said phase jitter for isolating a phase-jitter componenttherein, means for multiplying said phase-jitter component by the outputof said local oscillator to form a partial in-phase demodulating signal,means for combining said partial in-phase demodulating signal with aquadrature rotated output of said local oscillator to form a completein-phase demodulating signal having both frequency-offset andphase-jitter components, and an in-phase demodulator responsive jointlyto said received signal and to said complete in-phase demodulatingsignal to form a data output substantially free of frequency-offset andphase-jitter impairments.
 2. The synchronous carrier recovery systemdefined in claim 1 in which said transmission path for said phase-jittercomponent further comprises a bandpass filter whose bandwidth iscomparable to that of said phase-jitter component and an attenuator inseries with said last-mentioned filter for compensating for thetransmission level of said discrete carrier component.
 3. Thesynchronous carrier recovery system of claim 1 in which an integratoroperates on the output of said in-phase demodulator to derive therefroman output corresponding in magnitude to the transmission level of thecarrier-frequency component in said received signal and a subtractorjointly responsive to the outputs of said in-phase demodulator and saidintegrator removes the direct-current component from the output of saidin-phase demodulator.
 4. In combination with a vestigial-sideband,amplitude-modulated transmission system for data signals which includesa discrete carrier component shaped to exclude quadrature direct-currentenergy, said transmission system having a tendency to impart distortingfrequency-offset and phase-jitter components to received signals, asynchronous demodulating carrier recovery arrangement comprising anin-phase demodulator; a quadrature demodulator; means for applyingreceived data signals to said in-phase and quadrature demodulators; alocal carrier-wave source; a first control loop jointly controlled bysaid quadrature demodulator and said carrier-wave source to lock thefrequency of said carrier-wave source to the discrete carrier componentin received data signals, said first control loop having a narrowpassband including direct current comparable to the passband of saidfrequency-offset component; a second control loop jointly responsive tosaid quadrature demodulator and said carrier-wave source for trackingthe phase-jitter component of said received signal, said second controlloop having a passband not including direct current comparable to thepassband of said phase-jitter component; means included in said secondcontrol loop for multiplying together the output of said carrier-wavesource with said phase-jitter component thereby producing a demodulatingcarrier-wave with a superposed phase-jitter component; further meansincluded in said second control loop for attenuating said phase-jittercomponent therein in accordance with the amplitude of the discretecarrier component in said received signal and for applying saidattenuated phase-jitter component to said multiplying means; means forshifting the phase of the output of said carrier-wave source by 90electrical degrees to provide an in-phase demodulating carrier-wave;means responsive to the output of said multiplying means for adding saidsuperposed phase-jitter component to the in-phase demodulating carrierwave from said phase-shifting means; means for applying the output ofsaid adding means to said in-phase demodulator; means for integratingthe output of said in-phase demodulator to obtain the direct-currentcomponent of the demodulated received signal; means for substituting thedirect-current component obtained from said integrating means from thedemodulated received signal from said in-phase demodulator; and furthermeans for applying the direct-current output from said integrating meansto said attenuating means to control the effective level of attenuationthereat.